The present invention relates to a power supply apparatus, and in particular to a variable-capacitance power supply apparatus for storing charge to supply an instantaneous high power to a load.
Safety testing of circuits against excessive power requires producing and measuring such excessive power. FIG. 1 is a simplified schematic circuit diagram of a test system for testing short-circuit or breakdown characteristics of a DUT (device under test) 10, such as an insulated gate bipolar transistor (IGBT), a high current resistor, a motor coil or the like. The short-circuit test system supplies a high voltage across the output terminals of the DUT 10, such as between the collector and emitter of an IGBT, to measure the breakdown voltage thereof. The short-circuit test system includes a voltage source 11, a voltmeter 13, an ammeter 15, and a switch 12 for switching the voltage input from the voltage source 11. Where the DUT 10 is an IGBT, a pulse generator 14 supplies pulses to the gate of the IGBT to turn on or off the IGBT. Measurement of the characteristics of the DUT 10 by applying a high voltage thereto needs an appropriate high voltage source apparatus.
A commercially available AC power supply provides 50/60 Hz and 100 V power. One method of obtaining a DC voltage higher than 100 V is to rectify a step-up-transformed AC input to charge a smoothing capacitor. The peak voltage across the smoothing capacitor is equal to the maximum output voltage from a capacitor charger. In order to increase the peak output voltage it is necessary to increase the maximum output voltage of the capacitor charger. Such a high voltage output capacitor charger is expensive in construction.
In a short-circuit test an excessive short-circuit current flowing through DUT 10 may destroy or cause damage to other associated circuit elements. The total energy W stored in a capacitor depends on the amount of charge Q stored on the capacitor, i.e., W=(1/2)QV where V is the voltage across the capacitor. The amount of charge Q is proportional to the capacitance C, i.e., Q=CV where C is the capacitance of the capacitor. The quantity CR is a time constant for discharging the smoothing capacitor, where R is the resistance of the load. The smaller the capacitance C, the faster the current flowing through the load is reduced.
Thus, what is desired is a variable-capacitance power supply apparatus having an inexpensive structure for variably supplying a desired high power to a load by selecting an appropriate total capacitance for storing charge.